Quotes

"Don't mistake me. I do love the ladies. It's just lately I've been wondering what it'd be like to share the slaughter of innocents with another man. Don't think that makes me some kind of a deviant, do you?"
--Angelus to Spike, Angel the Series: "Destiny"

ILLYRIA: A corrupted ruler on such a path sees treachery and betrayal all around him. He cannot suffer intimates and will eventually turn against them.SPIKE: Guess I don't have to worry about that, 'cause Angel and me have never been intimate. Except that once...
--Angel the Series: "Power Play"

Canon Relationship

On the show, Angel and Spike's relationship was portrayed primarily as antagonistic. There is no canon evidence of an actual romantic/sexual relationship between the two apart from the small mention in season 5 of Angel: the Series episode Power Play (see quote above).

Their relationship is based on a lot of jealousy for the women they are involved with (first Drusilla, then Buffy) and anger at real or perceived crimes against one another.

- more on season 5?
- Comics

Fandom & Fanon

The Sire Debacle

In BtVS season two episode School Hard, during their first confrontation, Angel pretends to be Angelus and tries to get close to Spike while using Xander as bait. When Spike realizes the trap he reacts violently and ends up speaking one of the most influencial sentence in the history of this pairing:

Because of that line, the fandom widely accepted the fact that Angel was the one who turned Spike into a vampire. This resulted in a lot of fanfiction where Spike and Angel shared a Sire/Childe bond, could feel each other's emotions or something similar along those lines.

When it was revealed in season four of Buffy that Spike was actually turned by Drusilla, the popular consensus among the Angel/Spike shippers was that while Drusilla did the actual turning it was Angelus who trained and mentor Spike, resulting in the two of them developing what should be a Sire/Childe relationship.

SPIKE: Drusilla sired me, but you.. you made me a monster.
--Spike to Angel, Angel the Series: "Destiny"

Another (just as popular) approach is to simply ignore the canon and write as if Angel turned Spike.[1]

Common Fanfic Tropes

One of the common fanfic tropes is for Spike to turn to Angel when he needs help or is desperate and Angel stepping in and helping and comforting Spike. When this trope is used, the Sire/Childe bond is often at play as well.

Very often Angel will also start referring to Spike as Will (or Wil), especially if the fic uses Spike after he got his soul back.